CXCR4, a G-protein-coupled receptor, and its naturally occurring ligand, stromal cell-derived factor-1 (SDF-1; CXCL12), are a chemokine receptor-ligand pair. CXCR4 is constitutively or over-expressed in a wide variety of human cancers (Table 1). SDF-1, the only known ligand of CXCR4, is highly expressed in tumor microenvironments, as well as in bone marrow, lung, liver, and lymph nodes, i.e., organ sites most commonly involved in tumor metastasis. CXCR4/SDF-1 interaction plays important roles in multiple stages of tumorigenesis, including tumor growth, invasion, angiogenesis, and metastasis (Furusato, et al., Pathology International 2010, 60, 497-505). The CXCR4/SDF1 axis also serves a role in attraction multiple leukocyte subsets and stimulation B cell production and myelopoeisis, all of which are implicated in autoimmune diseases (Chong and Mohan, Expert Opin. Ther. Targets 2009, 13(10), 1147-1153). In view of the involvement of CXCR4/SDF-1 in these serious diseases, the CXCR4/SDF-1 pathway may be an attractive therapeutic target (Tamamura and Fujii, Expert Opin. Ther. Targets 2005, 9(6), 1267-1282).
TABLE 1CXCR4+ Expressing Tumors.LeukemiasBrian tumorsSmall cell lung cancerBreast cancerProstate cancerRhabdomyosarcomaNeuroblastomaWilms' tumorHepatoblastomaOvarian cancerCervical cancerOsteosarcomaSources: Kucia, et al., Stem Cells 2005, 23, 879-894; Furusato, et al. 2010, 60, 497-505; Retz, et al., Int. J. Cancer 2005, 114, 182-189.
Tec kinases represent the second largest family of nonreceptor tyrosine kinases and are activated in response to cellular stimulation by antigen receptors, integrins, growth factors, cytokines and G protein-coupled receptors (Qiu and Kung, Oncogene 2000, 19, 5651-5661). The mammalian Tec family consists of five members: Tec, BTK, Itk/Emt/Tsk, Rlk/Txk, and BMX/ETK (Mano, Cytokine Growth Factor Rev., 1999, 10, 267-280). With some exceptions, Tec kinases are expressed primarily in cells of hematopoietic lineages.
BTK (Bruton's Tyrosine Kinase) is a Tec family non-receptor protein kinase expressed in B cells and myeloid cells. BTK is composed of the pleckstrin homology (PH), Tec homology (TH), Src homology 3 (SH3), Src homology 2 (SH2), and tyrosine kinase or Src homology 1 (TK or SH1) domains. Each of these domains has the potential to interact with a plethora of proteins critical for intracellular signaling. Moreover, functional association of BTK with many of its partners is crucial for its activation and regulation. BTK is a metalloprotein enzyme requiring Zn2+ for optimal activity and stability (Mohammed, Immunol. Rev. 2009, 228, 58-73). The function of BTK in signaling pathways activated by the engagement of the B cell receptor (BCR) in mature B cells and FCER1 on mast cells is well established. Functional mutations in BTK in humans result in a primary immunodeficiency disease (X-linked agammaglobuinaemia) characterized by a defect in B cell development with a block between pro- and pre-B cell stages. The result is an almost complete absence of B lymphocytes, causing a pronounced reduction of serum immunoglobulin of all classes. These findings support a key role for BTK in the regulation of the production of auto-antibodies in autoimmune diseases.
BTK is expressed in numerous B cell lymphomas and leukemias. Other diseases with an important role for dysfunctional B cells are B cell malignancies, as described in Hendriks, et al., Nat. Rev. Cancer, 2014, 14, 219-231. The reported role for BTK in the regulation of proliferation and apoptosis of B cells indicates the potential for BTK inhibitors in the treatment of B cell lymphomas. BTK inhibitors have thus been developed as potential therapies for many of these malignancies, as described in D'Cruz, et al., OncoTargets and Therapy 2013, 6, 161-176.
BTK is expressed in most hematopoietic cells except T cells and in osteoclasts but not in osteoblasts. The PH-TH domain of BTK has been shown to bind to βγ, Gαq and Gα12 subunits of heterotrimeric G-proteins and the association results in elevation of kinase activity. Thus BTK plays an important role in G protein signaling, including signaling downstream of CXCR4 (de Gorter, et al., Immunity 2007, 26, 93-104; Ortolano, et al., Eur. J. Immunol. 2006, 36, 1285-1295) and makes an attractive target for inhibition of CXCR4 signaling.